1. Field of Invention
The invention relates to glass break detectors and more specifically to such detectors that convert acoustic and other atmospheric waves into electrical signals for analyzing characteristics that represent breaking glass. Still more specifically, the invention relates to detectors that react to initiate an alarm only when the frequency, amplitude and temporal sequence of the electrical signals correspond to those associated with breaking glass.
2. Description of the Prior Art
Recent improvements in glass break detectors rely on the presence of selected sonic and subsonic frequencies expected to occur in a predetermined temporal sequence representing the events that accompany breaking glass.
One approach relies on the occurrence of a low frequency thump at the moment the glass breaks, followed by a brief silence and a high frequency tinkling caused by the broken fragments hitting each other and falling to the floor. False alarms are reduced by requiring detection of high and low frequency components in the expected order and separated by a short time interval. This approach is disclosed, for example, in Davenport U.S. Pat. No. 4,668,941, issued May 26, 1987.
Other approaches sense structurally transmitted vibrations in combination with sound and other atmospheric waves. The structurally transmitted component is combined with the atmospheric component in a time-dependent function to reduce false alarms. Marino et al,. U.S. Pat. No. 5,117,220, issued May 26, 1992, discloses an example.
Still other approaches translate energy developed by breaking glass into electrical signals having low and high frequency components. The respective components must occur within specified time windows and above predetermined amplitudes before the detector will sound an alarm. Yanagi et al. U.S. Pat. No. 4,091,660, issued May 30, 1978, discloses one example using a piezoelectric element mounted on the glass. Smith et al. U.S. Pat. No. 5,192,931, issued Mar. 9, 1993, discloses another example substituting an acoustic transducer, such as a microphone, for the glass mounted piezoelectric element. The microphone senses atmospheric waves including a low frequency positive wave generated by an inward flex of the glass and high frequency waves generated by the glass breaking. The signals must occur in a predetermined order, and the alarm is inhibited if the high frequency waves are preceded by negative-going low frequencies that typically would accompany the opening of a door.